This invention relates to a fuel injector for use in delivery of fuel under pressure to a cylinder of an associated compression ignition internal combustion engine. In particular, the invention relates to a fuel injector of the type suitable for use in a fuel supply system of the common rail type, the injector being actuable to permit fuel to be delivered to the cylinder of the associated engine from the common rail, the common rail being charged with fuel under pressure by an appropriate high pressure fuel pump. A plurality of similar injectors are arranged to receive fuel from the common rail.
It is known to control the operation of such a fuel injector by using a valve to control the fuel pressure within a control chamber, the fuel pressure within the control chamber acting upon a surface associated with the needle of the injector to apply a force to the needle urging the needle towards its seating. In order to ensure that injection terminates quickly upon closing the valve, it is known to use a flow restrictor to limit the fuel pressure acting on the needle and urging the needle away from its seating.
According to the invention there is provided a fuel injector for use in a common rail fuel system, the injector comprising a valve needle spring biased towards a seating, the valve needle including at least one thrust surface orientated such that the application of fuel under pressure thereto applies a force to the needle urging the needle from its seating, a piston slidable within a bore and defining, with the bore, a control chamber, the fuel pressure within the control chamber being controlled by a control valve, the fuel pressure within the control chamber applying a force to the piston which is transmitted to the valve needle urging the needle towards its seating, wherein the effective area of the piston is greater than the effective area of the thrust surface(s) of the needle.
Such an arrangement is advantageous in that the use of flow restrictors restricting the rate of fuel flow towards the seating can be avoided, the difference in area producing the biasing force necessary to cause rapid termination of injection.
The force is conveniently transmitted from the piston to the injector needle through a thrust pin of short axial length. Reducing the length of the thrust pin is advantageous as flexing of the thrust pin, in use, is reduced. Where a relatively long thrust pin is used, the flexing of the thrust pin results in jerky movement of the injector needle and hence in poor injection quality.